Comparaison des effets de la contrainte hydrique sur la croissance, la conductance stomatique et la photosynthèse de jeunes plants de chênes méditerranéens (Quercus suber, Q. faginea, Q. coccifera) en Tunisie

Comparison of the water stress effects on stomatal conductance, photosynthesis and growth of Mediterannean oak seedlings (Quercus suber L., Q. faginea, Q. coccifera) in Tunisia. Two evergreen oaks (Quercus suber L., Q. coccifera) and one deciduous oak (Q. faginea Willd.) were subjected to drought in a nursery in Tunis. Six- and 18-month-old seedlings were grown in pots and underwent two cycles of drought by withholding water supply. Predawn and midday leaf water potentials, stomatal conductance, photosynthesis and plant transpiration were recorded. An analysis of root and shoot biomass, shoot-to-root ratio and leaf area was also performed. Our results showed a higher reduction of shoot-to-root biomass ratio of Q. faginea compared to Q. coccifera under water limitation. The stomatal conductance decrease, related to the decrease of predawn leaf water potential, occurred earlier with Q. faginea. Q. coccifera kept its stomata partly open at a water potential of -3.0 MPa. Q. suber behaved somewhat intermediate. Photosynthesis and stomatal conductance were correlated and showed a midday depression. Our results indicate that more ecophysiological studies are required to take into account seedling and leaf ages during the juvenile stage for a better understanding of the water stress responses of these species and regeneration problems of oaks.